1. Field of the Invention
This invention relates generally to measurement circuits used in automatic test equipment, and, more particularly, to measurement circuits employing accurate pedestal sources.
2. Description of the Related Art
A fundamental requirement of automatic test systems is the ability to measure accurately electronic signals from devices under test. A variety of circuit topologies have been devised for this purpose.
FIG. 1 shows a high-level example of a measurement circuit that employs an accurate pedestal source 112. The pedestal source is generally programmable and is adjusted to produce an accurate voltage VPED approximately equal to an expected differential input voltage HI−LO of the measurement circuit. A summing circuit 114 adds a low side (LO) of the measurement circuit to VPED, and a differential amplifier 110 measures the difference between a high side of the measurement circuit (HI) and the output of the summer 114 (LO+VPED). The output of the differential amplifier is then customarily converted to a digital signal by an analog-to-digital converter 116.
The measurement circuit reads the value, G*[(HI−LO)−VPED], where G is the gain of the differential amplifier 110. This reading corresponds to the difference, or “residue,” between the actual input voltage HI−LO and the programmed expected input voltage, VPED. This reading can be converted to a measurement of input voltage, HI−LO, by and adding a numeric value that corresponds to VPED to the reading (and correcting for gain). Thus, for example, if the measurement circuit were to read 2.4 volts with G=100 and VPED=1 volt, the actual input voltage implied by these conditions would be 1 volt+2.4 volts/100=1.024 volts.
The measurement circuit can be made to be very precise because VPED can be precisely characterized and can be very stable. We have recognized, however, that the measurement circuit has errors that may adversely affect its performance. For example, offset voltage errors in the differential amplifier 110 induces errors in the measurement circuit. Because the residue is generally a small voltage, it is desirable to operate the differential amplifier 110 at high gain, to assure that the residue is large enough to be readily measured. As is known, however, offset errors of differential amplifiers grow proportionally larger as gain is increased.
It would be desirable for measurement circuits employing pedestal sources to produce large residue signals without being so adversely affected by errors in the differential amplifier.